Surge protectors are used in a wide variety of commercial and consumer applications to protect AC power distribution systems and various types of electrical and electronic equipment that receive operating power from such systems. AC power distribution systems and the electrical loads that receive power from such systems can be subjected, often repeatedly, to a potentially damaging and/or hazardous condition commonly referred to as a "power surge." A power surge is characterized by a sudden and dramatic transient increase in the magnitude of voltage one would normally expect to encounter at a given point in the AC power distribution system. Typically caused by lightning strikes or equipment failures of some kind, power surges can damage or destroy electrical insulation systems as well as motors, switches, control devices, computer systems, and all other types of electrical or electronic equipment. Moreover, such damage may result in fire and/or electrical shock hazards capable of causing death, serious injury, and/or ancillary property damage extending beyond the limits of the electrical system or equipment subjected to the power surge itself.
In order to prevent or mitigate the effects of power surges, various types of surge protectors are well known. Surge protectors typically include one or more devices commonly referred to as transient voltage surge suppressors (TVSSs), or more recently, as surge protection devices (SPDs). As used herein and in the appended claims, the term "surge protection device" is to be construed broadly to refer to any type of device which, in normal operation, exhibits a relatively high electrical impedance, but which, upon being subjected to a voltage of sufficiently high magnitude, exhibits a significantly lower impedance and conducts electrical current relatively readily. Non-limiting examples of various types of SPDs include varistors, silicon avalanche diodes, zener diodes, selenium cells, gas discharge tubes, and high voltage capacitors, of which the metal oxide varistor (MOV) is favored for many low voltage applications, such as in AC power distribution systems having normal, nominal operating voltages of about six hundred volts AC (600 VAC) or less.
In addition to SPDs, surge protectors may optionally include one or more thermal or overcurrent protectors. As used herein and in the appended claims, the term "thermal protector" is to be construed broadly to refer to any type of device which, in normal operation, exhibits a relatively low electrical impedance, but which, upon being subjected to temperatures above a temperature threshold, exhibits a significantly higher impedance and substantially prevents the flow of electrical current. Non-limiting examples of various types of thermal protectors include thermal fuses, bimetal thermostats, thermal cutoffs, thermal cutouts, and thermal links.
Thermal protectors are constructed and function in a variety of ways. In some implementations, a mechanically biased element, such as a spring or a flexed piece of metal, is soldered to one end of the device to be protected (e.g., an SPD). When the protected device heats up to a melting temperature of the solder, the solder melts and the biased element recoils or reverts back to its unbiased state, opening the circuit path and abating overheating of the protected device. One such thermal protector, a flexed piece of metal, is described in U.S. Pat. No. 5,790,359. Another such thermal protector, a "MICROTEMP" thermal fuse that is commercially available from Therm-O-Disc, Inc. of Mansfield, Ohio, is mentioned briefly in U.S. Pat. No. 5,621,602. Although both thermal protectors provide desired thermal protection of SPDs, they do so at the expense of additional elements and cost. That is, such thermal protectors require both a mechanically biased member and solder to connect the mechanically biased member to the protected device, such that when the solder melts the biased member is released to open the circuit.
In other thermal protector implementations, such as bimetal thermostats, a composite piece of metal is thermally coupled to one of the devices to be protected. The composite piece of metal is fabricated using two metals (hence the name "bimetal") characterized by differing coefficients of thermal expansion. As the protected device heats up, the composite piece of metal bends due to the interaction of the two pieces of metal expanding at different rates, such that upon the protected device reaching a temperature threshold, the composite piece of metal is bent sufficiently to open the circuit. Although providing some thermal protection, bimetal thermostats only temporarily open the circuit because as the protected device cools (e.g., after the circuit has been opened for a short length of time), the composite piece of metal also cools and bends back to its original position, thereby permitting current flow. Permitting such current flow could be catastrophic when the reason the temperature of the bimetal thermostat rose in the first place was the resistive heating of a failing SPD. In addition, bimetal thermostats are relatively expensive due their unique bimetal compositions.
FIG. 1 is a block diagram of a typical prior art electrical surge protector 100 for use in an AC power distribution system. The surge protector 100 includes a first SPD 107 connected between the neutral conductor 103 and the ground conductor 105 of the AC power distribution system, a second SPD 108 connected between the neutral conductor 103 and one end of a fuse 113, and a third SPD 109 connected between the ground conductor 103 and one end of the fuse 113. The fuse 113 provides overcurrent protection to SPDs 108 and 109, and couples SPDs 108 and 109 to the line conductor 101 of the AC power distribution system.
One problem with the surge protector 100 of FIG. 1 is that when the SPDs 107-109 reach the ends of their useful lives, they may cause undesirable effects in the AC power distribution system. An SPD's end-of-life can be caused by a variety of phenomena, such as temporary or permanent failure of the neutral conductor 103, improper installation of the SPD, lightning strikes, common voltage transients, and normal device fatigue. Undesirable effects in the AC power distribution system that may be caused by one or more SPDs reaching their end lives include removal of a branch circuit from the distribution circuit, removal of the distribution circuit from the service entrance, shutdown of a building's power system, high neutral-to-ground current flow, nuisance tripping of a facility's ground fault protection and fire.
As an SPD reaches the end of its useful life, the SPD (e.g., SPD 108) can become a low resistance (instead of its typical high impedance) at normal operating voltages and begin heating very quickly. If the resistance of the SPD 108 is lower than normal, but high enough to draw an amount of current just below the actuation rating of the fuse 113, the SPD 108 will continue to heat, eventually leading to a thermal runaway condition that could result in melting or arcing of the SPD 108. In addition, thermal runaway is even more likely for the SPD 107 between neutral 103 and ground 105 because the fuse 113 does not provide overcurrent protection for excessive neutral-to-ground currents.
FIG. 2 is block diagram of another typical prior art electrical surge protector 200 that attempts to overcome some of the limitations of the surge protector 100 of FIG. 1. As shown, the surge protector 200 of FIG. 2 includes three circuit paths. The first circuit path couples the line conductor 201 to the neutral conductor 203 and includes a first fuse 213, a first thermal cutout 215, and a first SPD 207 electrically in series with one another. The second circuit path couples the line conductor 201 to the ground conductor 205 and includes a second fuse 217, a second thermal cutout 219, and a second SPD 209 electrically in series with one another. The third circuit path couples the neutral conductor 203 to the ground conductor 205 and includes a third fuse 221, a third thermal cutout 223, and a third SPD 211 electrically in series with one another. Each fuse 213, 217, 221 provides overcurrent protection to a respective one of the SPDs 207, 209, 211. In an analogous manner, each thermal cutout 215, 219, 223 provides thermal protection to a respective one of the SPDs 207, 209, 211. Thus, in contrast to the surge protector 100 of FIG. 1, when an SPD (e.g., SPD 211) reaches the end of its useful life, overheating of the SPD 211 is abated by the respective thermal cutout 223, which is thermally coupled to the SPD 211. When the thermal cutout 223 reaches a predetermined temperature, the thermal cutout opens and no current flows through the SPD 211. In addition, the surge protector 200 of FIG. 2 provides both thermal and overcurrent protection to the SPD 211 coupled between the neutral conductor 203 and the ground conductor 205.
Although the surge protector 200 of FIG. 2 appears to solve all the problems of the surge protector 100 of FIG. 1, such is not the case. One problem that exists in the surge protector 100 of FIG. 1, and remains in the surge protector of FIG. 2, is the difficulty of detecting a fault in the neutral-to-ground circuit path. During normal operation of the AC power distribution system, the voltage present from neutral 203 to ground 205 can vary from zero volts AC (0 VAC) to forty volts AC (40 VAC). Consequently, simply sensing the voltage from neutral 203 to ground 205 to detect a failure of an element (e.g., SPD 211, thermal cutout 223, or fuse 221) in the neutral-to-ground circuit path is uninformative. An in-line current meter could be added to the neutral-to-ground circuit path, but adding such circuitry adds cost and complexity to the surge protector 200 and such circuitry may be damaged by voltage transients.
In addition to sharing surge protector 100's difficulty of detecting a fault in the neutral-to-ground circuit path, the surge protector 200 of FIG. 2 has considerably more components than its FIG. 1 counterpart, and accordingly is more expensive to manufacture. That is, although the circuit 200 of FIG. 2 provides much more thermal and overcurrent protection than does the circuit 100 of FIG. 1, it does so at the cost of more parts. The surge protector 200 of FIG. 2 not only requires three thermal protectors (i.e., thermal cutouts 215, 219, 223) not present in the surge protector 100 of FIG. 1, but also requires three overcurrent protectors (i.e., fuses 213, 217, 221), which is two more than the single overcurrent protector (i.e., fuse 113) used in the surge protector 100 of FIG. 1.
Therefore, a need exists for an apparatus and method for surge protecting an electrical load connected to an AC power distribution system that provide thermal and overcurrent protection for the SPDs, while utilizing a minimal number of components to do so. There is a further need for such a surge protection apparatus and method that also facilitates simple detection of a fault in the neutral-to-ground circuit path. There is a still further need for an economical thermal protector for use in such a surge protection apparatus and method that permanently reduces current flow when the thermal protector reaches an undesired temperature level.